To convert forward and backward movements into a rotational movement having a constant direction of rotation, transmission units are known from European Patent Publication No. 0 004 205A1 and U.S. Pat. No. 3,301,574, such transmission units having reverse motion and operating with direction-reversible adjusting ratchet mechanisms. However, these transmission units are disadvantageous because they only utilize one of the two driving movements for the rotational movement of the drive and they all include an idling stroke.
The invention seeks to provide a light-weight transmission unit of the above-described type which is as small as possible and in which the direction of movement is to be selectively converted into forward and backward movements without the need for idling to be bridged.